The present invention relates to a shrink-wrapped package prepared using a polyester film containing an ethylene terephthalate unit, the shrink-wrapped package having a portion of sealing and cutting (fusion sealing and cutting) which has an excellent impact resistance. The present invention also relates to a process for shrink-wrapping an article or articles with said polyester film.
Films for shrink wrapping, especially those for shrink-wrapping a stack of articles, are required to be free from wrinkles or blots after shrinkage, capable of tightly uniting the articles, and excellent in impact resistance at the portion of sealing and cutting. Conventionally, shrinkable films of polyvinyl chloride or polyolefin are widely utilized.
Although shrinkable films of polyvinyl chloride have excellent shrinking properties and satisfactory impact resistance at the portion of sealing and cutting, they have hygienic problems owing to plasticizers, heat stabilizers and processing aids contained therein, and problems with thermal disposal because they contain chlorine. Further, heat shrinkable films of polyolefin are excellent in impact resistance at the portion of sealing and cutting and free from problems with thermal disposal, but they need to be shrunk at a high temperature in order to obtain good finish. Moreover, films of polyolefin are likely to loosen after shrinkage and thus difficult to tightly unite articles. Also, they are limp and unsatisfactory in transparency.
Shrinkable films of polyester have stiffness, good transparency, excellent shrinking properties and capability of tightly uniting articles, and are free from problems with thermal disposal. The only disadvantage of the polyester films is that they have a poor impact resistance at the portion of sealing and cutting.
Japanese Examined Patent Publication No. 29139/1989 discloses a technique for improving the impact resistance of a sealed portion. This technique utilizes as a packaging material a heat shrinkable polyester film having a thermal shrinkage percentage of at least 10% in each of the machine and transverse directions, as measured after dipping in hot water at 100xc2x0 C. The heat shrinkable polyester film is prepared by orientating a polyester sheet at least 2.5 times in each of the machine and transverse directions and thermally relaxing the orientated sheet by 5 to 30% in at least one direction. The film is provided with a sealed portion having an impact strength of at least 10 kg-cm to package articles, and then thermally shrunk.
In this technique, however, the sealing method is limited to impulse heat sealing. Moreover, although the publication states that the film can be heat sealed when a thermal adhesive resin is applied to the film, the disclosed film, when subjected to sealing and cutting, has a low impact resistance at the portion of sealing and cutting, and thus cannot be put into practical use.
When a stack of articles is to be shrink-wrapped, generally employed is sealing and cutting using ultrasonic waves, high frequency or nichrome wire. This is because sealing and cutting enables continuous sealing, thus increasing the processing speed, and produces substantially no overlapping sealed portion so that the resulting packages have good appearance and a pile of the packages is unlikely to collapse even if the articles wrapped in the packages are of light weight.
In contrast, impulse heat sealing or heat sealing is conducted intermittently and thus slow in processing speed. Further, overlapping sealed portions with a width of 5 to 20 mm are usually produced, so that the resulting packages have poor appearance and a pile of the packages is liable to lean and collapse when the articles wrapped in the packages are of light weight.
An object of the present invention is to provide a shrink-wrapped package which is prepared using a polyester film and which has a portion of sealing and cutting having an excellent impact resistance.